10 CLASSIFICATION. 



mals, and that, consequently, the immense destruction of human life 

 which has resulted from its parasitic invasion of successive genera- 

 tions was designed when it was created. The opposite view is sup- 

 ported by numerous facts which show that these low organisms, like 

 those higher in the scale, are subject to modifications as a result of 

 changed conditions of environment, and that such modifications, in 

 the course of time, may become well-established specific characters. 



Again, the bacteria may be grouped into aerobic and anaerobic 

 species. This is a very important distinction, which was first estab- 

 lished by Pasteur, who found that certain bacteria will only grow 

 when freely supplied with oxygen, while others absolutely decline to 

 grow in the presence of this gas. The latter, which are spoken of as 

 strict anaerobics, may be cultivated in a vacuum or in an atmo- 

 sphere of hydrogen. Those species which grow either in the pre- 

 sence of oxygen or when it is excluded are called facultative an- 

 aerobics. 



Certain bacteria produce a peptonizing ferment which has the 

 power of liquefying gelatin. This has led to the classification of 

 those microorganisms of this class which grow in Koch's flesh-pep- 

 tone-gelatin as liquefying and non-liquefying bacteria. 



Again, we speak of them as motile or non-motile. 



It is evident that these biological characters, although all-im- 

 portant in the definition of species, cannot serve us in an attempt to 

 establish natural genera ; for the lines are not sharply drawn between 

 the saprophytes and the parasites, the aerobics and the anaerobics, 

 etc. , inasmuch as we have facultative parasites and facultative an- 

 aerobics which we cannot include in either class, and which yet do 

 not form a distinct class by themselves. We therefore adhere to the 

 morphological classification, although this is open to criticism. For 

 example, among the rod-shaped organisms which we call bacilli and 

 describe under the generic name Bacillus there are some which 

 multiply by binary division only, while others form endogenous re- 

 productive bodies known as spores. Certainly so important a differ- 

 ence in the mode of reproduction should be sufficient to separate 

 these rod-shaped organisms into two natural groups or genera. 



As heretofore stated, the German bacteriologist Hueppe has at- 

 tempted a classification based upon the mode of reproduction, in 

 which he makes two groups, or " tribes," one in which reproduction 

 occurs by the formation of endogenous spores " endospores " the 

 other in which it occurs by the formation of " ar thro spores." ' The 

 latter group includes all of those bacteria in which no other mode of 

 multiplication is known than that by binary division, which is com- 

 mon to all. In the present state of our knowledge this classification 

 1 An account of this mode of reproduction is given on page 19. 



